This is UB under std::string_view, but StringData accepts it.
To convert to std::string_view we would need to stop relying on this.

Add an invariant to the body of this constructor to ensure that we don't use it.

Possibly create a helper function that can be used where we might want to accept null pointers. Which could be something like:

constexpr StringData nullSafeStringData(const char* s) noexcept {
    return s ? StringData{s} : StringData{};
}

Add a

to the function declaration. This could unlock some small incidental optimizations as a side effect.